In this program we shall employ the methods of psychophysics and of neuromagnetism to identify the site or sites of lesions that underly subjective idiopathic tinnitus. The neuromagnetic field of the auditory cortex is capable of revealing features of its tonotopic map. Functionally discrete regions of the cortex can be localized within a few millimeters. It is also possible to localize sub-cortical sources of fields from their distribution external to the scalp. We plant to extend these methods to a study of the sources of auditory evoked fields in the brain stem and to the cochlea itself, where high-density currents are set up across the basilar membrane by acoustic stimuli. The methods of psychophysics will be employed to identify the physical parameters of stimuli that mimic tinnitus as it is actually experienced by the patient. We will modulate these stimuli to evoke cortical, subcortical and cochlear responses. These responses will be compared to those evoked by stimuli that do not mimic the tinnitus and are within the range of normal hearing of the patient. They will also be compared with responses obtained from normals. Criterion-free procedures will be employed to examine the effectiveness of various masking stimuli in detection of stimuli that mimic the tinnitus. We shall define an optimum masker - one that delivers the minimum amount of acoustic energy needed for effective masking. The physiological responses to stimuli that mimic the tinnitus will also be measured while a masking stimulus is present to determine if the masker "normalizes" abnormal responses to the stimuli when they are presented alone. We shall take account of the variability in the tinnitus as it is experienced by the patient in the conduct of these studies. Procedures will be devised to measure the tinnitus quickly and reliably in patients. The neuromagnetic measure is potentially useful in localizing the site of lesion and in providing an objective index of the efficacy of treatment.